The selective para-bromination of toluene, catalyzed by NaY zeolite in the presence of an epoxide, has been reported by F. de la Vega et al., J Chem. Soc., Chem. Commun., 1989, 653. The ratio of p- and o-isomers obtained was 98 to 2, but only about 10 to 13 percent of the toluene was converted. In experiments using no epoxide or in the presence of carbonates. the reaction proceeded to completion but the final para-selectivity was only 67%. The amount of zeolite catalyst used was about 0.09 gram per mmol of toluene. It was stated that adding a fresh batch of catalyst made the reaction resume with the same selectivity, but the final conversions obtained are not reported.
The para-selective bromination of benzene derivatives, such as toluene, using shape-selective zeolite catalysts, for example, NaY, HY, and NaX zeolites, has also been described by K. Smith et al., "Highly efficient para-selective bromination of simple aromatic substrates by means of bromine and a reusable zeolite," Chem. Commun., 1996, 467-468. In contrast to de la Vega, much larger amounts of zeolite were used in order to obtain complete conversion. When NaY zeolite was used in an amount of 0.55 gram per 0.85 mmol of toluene (0.65 gram per mmol), it was reported that 98% absolute yields of p-bromotoluene were obtained. When the reaction was carried out with fluorobenzene and NaY zeolite, only 92% of the desired p-bromofluorobenzene was obtained. The yield of para-brominated products which can be achieved is also indicated to depend upon the type of zeolite used. For example, NaY zeolite gave better conversion (yield) than using the same amount of a NaX zeolite in producing p-bromotoluene. Smith et al. claim that the zeolite catalysts can be regenerated by calcination, but it is not believed that calcination returns the zeolites to their original state. The selectivity incorporated by zeolite in the Smith et al. process does not offer any economical advantage over the available commercial methods (which can produce up to 90% of the desired p-bromofluorobenzene) for the following reasons:
1) Use of a very dilute reaction medium (about 1 wt % of the total reaction mixture) decreases plant throughput; PA0 2) A huge excess of zeolite is used (7 wt %, or a seven-fold excess over the substrate); and PA0 3) Low selectivity for the para-isomer is shown; para/ortho (p/o)=92/8. In any event, the relatively large amount of catalyst required to be regenerated would make this process too expensive to be practical from a commercial standpoint.
Fluorobenzene is a deactivated system towards electrophilic aromatic substitution and requires Lewis acid catalysis for the improvement of the rate of the substitution reaction. However, strong Lewis acid catalysts create fast reactions with lowerp/o ratios. Control of the reaction rate with catalyst and temperature are two methods reported for the improved selectivity. For example, Jacob Oren in EP 0 761 627 A1 describes the effect of temperature on the para-selectivity of the reaction of fluorobenzene with bromine. Selectivities as high as 97-98% were reported at temperatures between 0 to -20.degree. C., and even higher selectivities were achieved at temperatures between -17 and -65.degree. C. Use of such low temperature operations is not a preferred way of operating on a commercial basis.
It would be highly advantageous if a way could be found to produce bromoaromatic compounds in high yield and selectivity by process technology suitable for use on an industrial scale, while at the same time utilizing a shape-selective zeolite catalyst more efficiently than the processes described in the foregoing literature references. In the case of p-bromofluorobenzene production, such process technology would be particularly attractive if product with para/ortho ratios in the vicinity of 98/2 could be achieved without need for any operations conducted at temperatures below room temperature (i.e., below ca. 20.degree. C.). This invention is deemed to fulfill this objective most expeditiously.